0
$\begingroup$

I'm confused about the terminology and directions described in processing DNA to proteins in this paragraph.

RNA polymerase reads a DNA template in the 3’ to 5’ direction to synthesize RNA in the 5’ to 3’ direction. Transcription can occur in either direction, but not in either direction from a single promoter. RNA polymerase can only transcribe in one direction (5’ to 3’) with respect to reading one strand of the template DNA in the opposite direction (3’ to 5’). In general, each promoter recruits RNA polymerase to a particular site and indicates in which the direction RNA polymerase will move along the DNA. As RNA polymerase is moving, there will be only a single strand that can be used as a template to allow 5' to 3' synthesis of a transcript.

Sentence 1:

What's the difference between reading and synthesizing RNA?

Sentence 2:

What's the difference between synthesizing and transcribing?

Sentence 3:

Didn't this just contradict sentence 2?

Sentence 4:

If RNA polymerase is recruited at a site to and directed to "move along" the DNA, what is it doing when it "moves along" (reading? synthesizing? transcribing? wtf?).

Sentence 5:

My understanding is this is the "transcribed strand", not the "coding strand" of DNA, correct?

Question:

I don't understand this paragraph and the more I read it, the more confused I get. Could someone please clarify (preferable with visuals)?

$\endgroup$
  • $\begingroup$ I think if you search youtube for "rna transcription animation" you will find plenty of visual demonstrations. $\endgroup$ – Superbest Feb 4 '18 at 6:11
2
$\begingroup$

What's the difference between reading and synthesizing RNA?

What's the difference between reading and writing a book? Synthesizing RNA means producing an RNA based on a sequence in DNA. Reading it means processing RNA that is already produced. Reading also implies that protein will be produced based on the sequence in RNA.

What's the difference between synthesizing and transcribing?

"Synthesize" means to produce any chemical molecule. "Transcribe" is specifically the synthesis of RNA form DNA via a polymerase (as opposed to for instance inorganic synthesis of RNA de novo, as might be done in a chemistry lab).

Didn't this just contradict sentence 2?

It's just saying that genes can be oriented forward or in reverse. Reverse genes are on the opposite strand, so they are still 5' to 3' (the 5' ends of the two strands are on opposite ends of the DNA). But every gene must still have a promoter on its 5' side.

And actually, that bit is not quite true: There are bidirectional promoters that will cause transcription in both directions.

If RNA polymerase is recruited at a site to and directed to "move along" the DNA, what is it doing when it "moves along" (reading? synthesizing? transcribing? wtf?).

Usually, it will transcribe (so it will also synthesize, or "read"). But technically, RNA pols belong to a family of proteins which are "molecular motors", meaning that their fundamental activity is to move along DNA step by step like a little monkey climbing a ladder. With RNA pols, RNA synthesis is coupled to this movement, so each step on DNA also causes a letter to be added to the RNA they are making.

Also, technically the RNA pol is not only moving and making DNA, but also unwinding the two strands of DNA and separating them. But this may be outside the scope of your material.

My understanding is this is the "transcribed strand", not the "coding strand" of DNA, correct?

RNA comes out matching the sequence of the gene (except for T->U), and it is assembled through basepairing to DNA. If it paired with the strand the gene is on, the RNA would come out as the reverse complement of the gene. Instead it is assembled on the opposite strand, which is already the RC of the gene. RC of RC of a sequence is the same sequence. Thus we say RNA is transcribed from the "non-coding" strand, but keep in mind that these coding/non-coding assignments are arbitrary and only used by us humans, the cell does not have a special marking for it (beyond the 5'/3').

I don't understand this paragraph and the more I read it, the more confused I get. Could someone please clarify (preferable with visuals)?

It's very simple: DNA is a directed (because of 5'/3') string of ACTG. It is coupled to another, complementary string that runs opposite to it. The sequence on either strand may be meaningless, or parts of it may encode genes. Genes consist of a promoter, some sequence, and terminator. RNA pols can slide along either strand, but only in 5' to 3' direction, so they will slide in opposite directions on opposite strands, like cars on a two way street. When RNA pol encounters a promoter, it starts synthesizing an RNA: It separates the two strands, and lets single RNTPs basepair with the opposite-direction strand, when they do so it glues their phosphate backbones together. It keeps sliding and making RNA like this until it reaches another sequence, the terminator, at which point it stops assembling RNA and releases it. The RNA snaps off DNA and goes off on its merry way, the two strands of the DNA that were separated by RNA pol come together again.

Thus for every promoter-terminator pair, whatever sequence is in between them will be transcribed into an RNA. These are basically genes. These genes may be on either strand of DNA, and they must always be oriented along the 5' to 3' of the strand they are on. Thus genes on opposite strands will appear "reversed" with respect to each other.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.